The present disclosure generally relates to an injection device for injecting automatically a medicament and a method for detecting an occlusion in the injection device and, in particular, to an automatic injection device and a method for detecting an occlusion in the injection device using a force measurement unit for measuring an injection force.
Generally, it is insulin that is injected into the body of a patient, although other medicaments to be injected over a long period of time can also be injected by the device such as, for example, analgesics, pain killers or cancer drugs.
Automatic injection devices inject a predefined volume of a medicament into the body of a patient at predefined time intervals or delivery rates, respectively, in a continuous or quasi-continuous way. Typically, the delivery rate varies according to an individual schedule such as, for example, according to a pre-programmed circadian profile. Some devices allow the additional injection of larger drug boluses on demand. This volume is withdrawn from a reservoir, generally an exchangeable ampoule, via a pump mechanism and is injected through an injection needle placed in the patient's body. Automatic injection devices may be designed according to the syringe-driver principle. For carrying out an injection, a plunger that is received in a cartridge or ampule is linearly displaced in a controlled way via a typically motor driven pump mechanism with a piston rod that contacts and pushes the piston during operation. If an occlusion is present in such a system, the pressure in the injection system increases, since there is no drop in pressure provided by injections. As a result, a force that is to be applied in the pump mechanism increases over several unsuccessfully performed injections. Thus, a force measurement makes it possible to ascertain whether or not there is an occlusion. If an occlusion occurs, the patient is no longer supplied with a necessary medicament, thus causing potentially severe medical complications. Moreover, since the pump unit operates automatically at predefined time intervals, the pressure in the ampoule and in the feed lines to the patient's body increases, which could cause damage to the injection device. A still greater problem is that, with increasing pressure, the occlusion may eventually break up abruptly and the patient may then receive too large a quantity of the medicament. With a measurement unit that determines an injection force necessary for discharging the medicament, it is possible to ascertain whether an occlusion is present.
For example, such an injection device in which the volume to be injected is discharged by a piston that can be advanced by a spindle driven by an electric motor is known. In order to detect an occlusion during injection, in a first method, a maximum force threshold is predefined and, if it is exceeded, an occlusion alarm is triggered. In a second method, a force increase in a plurality of force values for discharging the injection volume is recorded. If no occlusion is present, then, no increase in force is detectable over a period covering a plurality of injections; only when an occlusion occurs is there an increase over a plurality of measurements. To determine the increase in force, fifteen force values, each determined at identical time intervals, are evaluated.
An automatic injection device with a measurement unit for measuring and storing force values associated with injections of a medicament is also known. An evaluation unit determines an injection occlusion from the measurement values, whereby a switching unit modifies automatically the time interval between individual force measurements depending on evaluation results provided by the evaluation unit. The time interval between the basal releases is maintained constant, e.g. about three minutes, independently of the time intervals for recording the force values. Initially, the force measurements are taken every three minutes in sync with the basal releases and stored over 30 minutes, and only if the evaluation unit determines based on these force measurements a potential occlusion, there is an extension of the time interval for measuring the force values. However, for very low infusion rates, the time interval between the single basal releases has to be increased for typical devices due to a limited injection volume resolution. The time interval between basal releases may, e.g., be extended to 30 minutes for a smallest basal delivery rate of 0.02 IU/h (International Units per hour). Here, the interval between basal releases is longer than the time interval for measuring the force values, resulting in multiple substantially identical measurements between consecutive injections, rather than measurements being in sync with the injections. As a consequence, an occlusion is not detected in this situation.
In addition, some infusion systems provide a check valve or pressure valve between the ampule and the feed line that only opens at a certain opening pressure. After insertion of an ampule into the injection device, the opening pressure has to be built up in the fluidic system by advancing the piston before the feed line can be filled with medicine in a so-called priming phase and injection can start. Once, built up, the fluidic pressure in the ampule is maintained substantially at the opening pressure. Until the opening pressure is built up, the force values, as determined by the measurement unit, continuously increase in a similar way as in case of an occlusion during regular operation. Therefore, such an occlusion detection needs to be deactivated for the time before the actual injections starts.
In some situations, however, an ampule with connected valve and feed line is removed from the device during operation for some time and subsequently re-inserted. When removing the ampule, the opening pressure of the valve, present before, is released. After re-insertion of the ampule, the opening pressure has to be built-up again. Since the device is in a regular operation mode and the occlusion detection is activated, the force increase while building-up the opening pressure may result in a false occlusion alarm.
When inserting an ampule into the device, the ampule is typically fully filled with the plunger in an end position. This, however, is not necessarily the case, for example, if an ampule is filled by a device user only partly, or if readily filled ampules of different filling volumes are commercially available. This is also the case, if an ampule is removed from the device and subsequently re-inserted as described above.
When inserting a new cartridge, a piston rod of the drive mechanism may first be moved forward in a delivery direction, until it contacts the piston of the ampule and building-up the opening pressure of the pressure valve and priming can start. This phase of device operation is referred to as “sniffing phase”. During the sniffing phase, the occlusion detection is favorably deactivated. If, however, an ampule is removed during operation and subsequently reinserted, building up the opening pressure and subsequent medicine delivery start immediately without sniffing phase and with the occlusion detection being activated.
Therefore, there is a need to provide an automatically operating injection device and an occlusion detection method which are more flexible and efficient with regards to different basal delivery rates and delivery rate intervals, particularly with regards to low basal delivery rates.